Brain tissue is constituted of neuronal cells which are able to communicate with each other via specific cellular structures named synapses. The exchange of signals between neurons in the synapses happens through neurochemical messengers named neurotransmitters, acting on specific target protein molecules, both post and pre-synaptic, referred to as receptors. Monoamines represent a family of small neurotransmitter molecules sharing common chemical features, and include serotonin (5-HT), dopamine (DA) and norepinephrine (NE).
Monoamine neurotransmitters are released into the synaptic cleft between neurons and interact with receptors present on the membrane of the target cells. The switch of the neurochemical signal occurs mainly by removal of the neurotransmitter molecules through other protein molecules referred to as monoamine transporters (SERT for 5-HT, DAT for DA and NET for NE). Transporters are able to bind neurotransmitter molecules and move them into the presynaptic terminals, this cellular mechanism referred to as re-uptake. Pharmacological inhibition of the re-uptake process can cause an increase of monoamine at synaptic level and as a consequence an enhancement of the physiological activity of neurotransmitters.
Serotonergic neurotransmission in the brain is mediated by a large family of receptors comprising both the G-protein coupled receptors and ligand-gated ion channels including 14 subtypes, and is involved in a vast variety of physiologic functions.
Compounds endowed of inhibitory properties at the SERT are predicted to have the ability to treat in mammals, including humans, a variety of disorders associated with this neural system, for example eating disorders, major depression and mood disorders, obsessive compulsive disorders, panic disorders, alcoholism, pain, memory deficits and anxiety.
Included among these disorders are disorders related to depression, such as pseudodementia or Ganser's syndrome, migraine pain, bulimia, obesity, pre-menstrual syndrome or late luteal phase syndrome, tobacco abuse, panic disorder, post-traumatic syndrome, memory loss, dementia of ageing, acquired immunodeficiency syndrome dementia complex, memory dysfunction in ageing, social phobia, attention deficit hyperactivity disorder, chronic fatigue syndrome, premature ejaculation, erectile difficulty, anorexia nervosa, disorders of sleep, autism, mutism or trichotillomania.
Major depression is an affective disorder, or disorder of mood, characterized by several symptoms including feeling of profound sadness, worthlessness, despair and loss of interest in all pleasures (anhedonia), recurrent thoughts of death, mental slowing, loss of energy, an inability to take decision, often associated with anxiety and agitation. These symptoms are persistent and can range from mild to severe.
The pathophysiology of major depression is poorly understood being a multifactorial syndrome and, due to this, several neurotransmitter systems have been implicated. However, it is generally believed that the disorder stems from a decrease in the synaptic concentration of monoamine neurotransmitters, mainly NE and 5-HT, in critical brain areas, leading to the “monoamine theory” of depression.
Several lines of preclinical and clinical evidence indicate that an enhancement of serotonin-mediated neurotransmission might be effective in the treatment of major depression and actually the selective serotonin re-uptake inhibitors (SSRIs) have come to dominate the therapy of depression over the last two decades. Fluoxetine, the first SSRI to be introduced, is the prototype of this group. Other members include Paroxetine, Sertraline, Fluvoxamine, Citalopram.
However, it is not clear exactly how these agents act to relieve depression. As with other classes of antidepressant, there is a lag of several weeks before the onset of the mood-elevating effect, despite the rapid blockade of the serotonin re-uptake. It is presumed that secondary adaptive changes must occur at serotonergic synapses after chronic administration of SSRIs i.e. down-regulation of release-regulating autoreceptors and increased neurotransmitter release. The delayed onset of anti-depressant effect is considered to be a serious drawback to currently used SSRIs. Moreover, although there is generally good tolerability of SSRIs, the elevation of 5-HT levels at central and peripheral synapses leads to stimulation of receptor subtypes like 5-HT2C and 5-HT3, which contributes to agitation and restless, along with gastrointestinal and sexual side-effects.
The success of the SSRIs rekindled interest in the development of selective norepinephrine re-uptake inhibitors (SNRIs) as potential antidepressants. A number of such compounds have been synthesized, e.g. Nisoxetine, Maprotiline, Tomoxetine and Reboxetine. Furthermore, many compounds, including old tricyclic antidepressants, have a mixed NET and SERT inhibition profile, like Imipramine and Amitriptyline (with SERT potency>NET) and Desipramine, Nortriptyline, and Protriptyline (NET potency>SERT).
The pharmacological manipulation of the DAT can in principle have the ability to elevate DA levels in the mesolimbic system, reversing the anhedonia that is a core symptom of major depression. A DAT inhibition component, in combination with a blockade of SERT and NET, can also have the ability to improve the lack of motivation and attention and enhance cognitive deficits seen in depressed patients. On the other hand, blockade of DAT has to be carefully managed in order to avoid potential reinforcing effects and abuse liability. However compounds with DAT inhibition in their pharmacology, such as Dexmethylphenidate, Methylphenidate and Bupropion, have been successfully marketed. Clinical studies indicate that patients with poor response to SSRIs benefit from combination therapy with agents that enhance dopaminergic tone. As a result, compounds with a strong SERT inhibiting activity combined with a well balanced NET blockade and moderate DAT inhibiting activity may therefore provide a replacement for current combination therapies for treating unresponsive patients, providing greater efficacy and therapeutic flexibility with a more rapid onset of anti-depressant effect.
Due to their valuable DAT inhibition, the compounds of the present invention are considered useful for the treatment of Parkinsonism, depression, obesity, narcolepsy, drug addiction or misuse, including cocaine abuse, attention-deficit hyperactivity disorders, Gilles de la Tourettes disease and senile dementia. Dopamine re-uptake inhibitors enhance indirectly via the dopamine neurones the release of acetylcholine and are therefore also useful for the treatment of memory deficits, e.g. in Alzheimers disease, presenile dementia, memory dysfunction in ageing, and chronic fatigue syndrome. Noradrenaline re-uptake inhibitors are considered useful for enhancing attention, alertness, arousal, vigilance and for treating depression.